Crystal oscillators were developed in the 1920's. There are a number of crystal oscillator circuits presently available for use as clock generators for digital circuits. Among the most well known are the series inverter chain, the Pierce and the Colpitts oscillators. The series inverter chain is generally considered to be inferior to the Pierce oscillator because of its tendency to either oscillate on crystal overtones, oscillate without a crystal or even fail to oscillate at all. Furthermore, the series inverter chain tends to be rather large, since it must be capable of driving the external pin capacitance. The Pierce oscillator, when properly designed, has none of these problems. Both oscillators, however, have the disadvantage that they require two package pins and several external components dedicated solely to the oscillator. Although either circuit may be adequate for general use, an oscillator requiring only one package pin and no external components is preferable.
In chip design, in order to keep the size of the package manageable, the number of external connections allowed is usually less than that desired. Often signals are multiplexed on the same pin instead of given separate pins to accommodate this need. Accordingly if one of the pins required for the oscillator could be eliminated, the chip designer would have greater design flexibility. Also with no external components, the oscillator would be immune to problems associated with their improper selection and placement.
The basic oscillator block, with an inverter 10, crystal Y.sub.1, and capacitors C.sub.A, C.sub.B, used in both the Colpitts and Pierce oscillators, is shown in FIG. 1. The capacitance C.sub.A, C.sub.B are external components which have high values on the order of 30 pF. This circuit has none of the problems of the series inverter chain. At resonance, the impedance of the crystal Y.sub.1 appears inductive, giving the necessary zero degrees loop phase shift for oscillations to occur. Parasitic elements along cannot have the same effect; therefore oscillations without the crystal are impossible. With only one stage of gain and a reactive feedback network which has low gain at all frequencies except the frequency of oscillation, the basic oscillator block does not have enough loop gain at the crystal overtones to allow oscillations at those frequencies. Also most important is the fact that all parasitic capacitances of the transistors, layout and external connections may be lumped into either C.sub.A, C.sub.B or the case capacitance of the crystal. Since these elements resonate together during oscillation, these capacitances are driven by the resonant current, and thus a high oscillator bias current is not necessary to drive large loads.
The basic oscillator block can be implemented as follows. By grounding node 3 and biasing node 2 with a current source, the Pierce oscillator is formed. Note that two pins (at nodes 1 and 2) are required to connect the crystal to the integrated circuit. Alternately, node 1 may be grounded and node 2 biased with a current source forming the Colpitts oscillator. In this configuration, the crystal Y.sub.1 is grounded on one side, and therefore only one pin of the integrated circuit need be dedicated to the oscillator and connected to node 2, as ground of the oscillator can be connected to the ground of the package. Unfortunately, in integrated form the Colpitts oscillator is difficult to bias.
The present invention is directed to overcoming the above disadvantages.